Dr. Omar M. Yaghi is widely known as the inventor of a new branch of chemistry, known as “reticular chemistry” which is defined as the “stitching of molecules together by strong bonds into extended structures.” This led his laboratory to design and produce new classes of crystals now famously known as Metal-Organic Framework (MOFs), Zeolitic Imidazolate Frameworks (ZIFs), Covalent Organic Frameworks (COFs) and Metal Organic Polyhedra (MOPs). Reticulated crystals hold many records, among them that having the highest surface area (5,640 m2/g for MOF-177) and the lowest density of any crystal (0.17 g/cm3 for COF-108). These materials have developed from basic science to applications in clean energy technologies, including hydrogen, methane and carbon dioxide capture and storage.
Generally speaking, MOFs are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional porous structures. Based on the combination of the building blocks, the length, the combination and the functionalization of the organic linker, a large variety of pore environments can be realized. Some of the interesting properties that MOFs exhibit include large surface areas, relative ease of tuning and the ability to functionalize for specific applications.
MOFs are of increasing interest due to their use as a highly selective and permeable membrane to separate small gas molecules, particularly CO2 from CH4. This separation is necessary for natural gas purification and CO2 capture, but it is difficult because the two molecules are very similar in size. Other possible applications of MOFs are gas purification, gas separation, gas storage and delivery, catalysis and sensors.
Pore size is very important in several of these applications. For example, MOFs with pore sizes greater than the target gases fail to exhibit high membrane selectivity. Selectivity is expected to be higher for structures where their pore size greatly inhibits the motion of larger molecules; i.e., forming a molecular sieve for target gases. Therefore, reliable knowledge of pore size characteristics would improve the selection of MOFs for their intended purpose.
However, pore size characteristics are not easy to collect. Pore size can be determined by gas adsorption porosimetry (e.g, using the Horvath-Kawazoe or Dubinin-Astakov calculation methods) or mercury intrusion porosimetry. However, these various methods are subject to bias, and even where reasonably accurate, manual determination of the pore size characteristics on a grand scale would be close to impossible. Therefore, one of the challenges involved in the efforts of creating real world applications using MOFs is the huge number of reported structures and the large times required to screen the structures due to the novel experimental techniques involved. Moreover, porosimetry only provides information on the total available pore volume. In many applications for chemical separations, it is also necessary to characterize the pore constrictions that control the movement of molecules within a porous material.
What is needed in the art is a method to make predictions regarding the pore size characteristics of MOFs and to use those predictions to screen large quantities of MOFs to isolate those which fall within a range of interest. Those MOFs of interest can be further analyzed by predicting their behavior as membranes based on molecular simulations. Embodiments of the invention, which provides a screening methodology for identifying MOFs for separation applications, meets these needs.